Method for producing rolled steel products



11, 1964 AKE TJELVAR FROLICH ETAL I 3,144,329

' v METHOD FOR PRODUCING ROLLED STEEL PRODUCTS Filed sept. 5, 1961United States Patent O 3,144,329 METHOD non PRODUCKNG noLLEn STEELrnonncrs Ake Tjeivar Friilich and John Arne Smedstam, flxelosund,

Sweden, assignors to Traiiiraktiebolaget Grangesberg- Oxelosund,Stockholm, Sweden, a joint-stock company of Sweden Filed dept. 5, 1961,Ser. No. 136,082 Ciaims priority, application Sweden, Sept. 6, 1960 7Claims. (Ci. 75203) This invention relates to a method for producingrolled steel products, preferably thin sheet, with low oxide contentfrom oxidic iron powder, preferably sponge iron relatively rich inoxide, by imparting to the oxidic iron powder either during itsmanufacture or thereafter a content of carbon.

The most types of sponge iron show an excellent property ofcompressibility, rendering it possible to compress porous sponge iron ina rolling mill or press such, that a product of high density andstrength is obtained. Even sponge iron in ground state can be compressedin the same manner. By sintering, after-pressing or rolling thecompressed products, the strength can be increased still more. Thismethod is applied in so-called powder-metallurgical manufacturingprocesses.

Neither sponge iron, however, nor sponge iron powder which are producedaccording to conventional reduction processes is adapted for use in theproduction of highgrade pressed or rolled products without having beensubjected to a special after-treatment. This is partially due to thefact that sponge iron almost always includes considerable amounts ofresidue iron oxides which may sometimes even exceed It is easy tounderstand, that such impurity reduces the strength of compressed spongeiron products and, in general, deteriorates the quality of the material.

In the production of iron powder for powder metallurgical purposes, thecontent of oxidic impurities usually is reduced by subjecting the spongeiron after grinding to a further reduction by hydrogen gas. Such areduction process involves, however, considerable costs, dependingpartially on the hydrogen gas price, nor is it, on the other hand, veryeffective, because the diffusion paths are relatively long and hightemperatures cannot be applied.

It is the object of the invention to eliminate the aforesaiddisadvantages or to reduce them to a minimum in those special cases wheniron powder is employed for producing rolled products, particularly thinsheet. The invention is substantially characterized in that thecarbonaceous and oxidic iron powder is first pressed to sheet bars, thatthe sheet bars are sintered at 6001200 C., so that their carbon andoxide contents are substantially reduced, whereafter the bars bycompression rolling are formed to billets which, finally, are subjectedto cold rolling in conventional manner.

If the production conditions of the iron powder are such, for exampleaccording to the H'tigan'as-method, that its carbon content can beadjusted, it is not necessary to admix further carbon to the ironpowder. If, however, the available iron-powder is produced according toa method, for example by reduction with hydrogen gas, which does notinclude the possibility of adjusting the carbon content, a certainamount of ashless carbonaceous material or pig iron powder, cementite oranother metallic carbide should be admixed, so that the carbon contentof the oxidic iron powder is adjusted to the oxide content of thelatter.

Thereafter, the iron powder is compressed.

According to the invention, the pressing of the powder to sheet bars iscarried out either with or without binding agents.

In this description, the term sheet bars refers to somewhat porousmetallic semifinished products adapted for rolling thin steel productsof substantially two-dimensional extension.

The sheet bars are then formed to solid, rolled steel products byannealing at 600-1200 C., preferably 900- 1150 C., and rolling. Theseoperations are repeated until the desired compression degree anddimensions are obtained. The compression to single sheet bars, whencompared with continuous compression, includes the considerableadvantage, that the subsequent treatment, for example sintering, can bedivided such, that one press is served by several sintering furnaces,etc. For a continuous strand, the entire material flow fro-m the millmust be handled by one furnace. In view of the time needed for thesintering process and the carbon-oxide reaction occurring during thisprocess, the sintering furnace must have considerable dimensions, if theproduction rate should be high.

The annealing is preferably carried out in a reducing protective gasatmosphere which may contain carburizing gases, for example carbonmonoxide and/ or methane. For rendering possible the adjustment of thecarbon content in the final product, it is of advantage to be able tocontrol the composition of the protective gas atmosphere.

The invention is illustrated by Way of an example in the followingdescription, reference being had to the accompanying drawing showing aflow-sheet of an embodiment of the invention.

The iron ore concentrate used had the following analysis values.

Chemical composition: Percent Fet t SiO 0.11 CaO 0.02 MgO 0.07 MnO 0.12Ti0 0.07 A1 0 0.10 V 0 0.27 P 0 0.021 S 0.003

The concentrate was reduced to spongeiron according to theHtiganas-process. The sponge iron tubes obtained were cleaned. Thesponge iron was crushed, coarseground and pulverized in a desintegrator,whereafter the material was subjected to dry magnetic separation. Thesesteps are represented by the areas 1a, 1b, 1c, and 1d on the drawing.The iron powder had the following chemical composition.

Chemical composition: Percent Fet t 9 8 Fe 97.2 SiO 0. 1 1 CaO 0.06 MgO0.08 TiO 0.06 A1 0 0.1 MnO 0.17 V 0 0.27 C 0.3 6 S 0.004 P 0 0.01 8

The concentrate included less than 0.2% rock, exclusive of lattice-boundmaganese and vanadium.

The iron powder, after magnetic extreme concentration, was then broughtto a press 2 and subjected to a pressure of about 1000 kg./cm so thatits density was 3 4. At this density, the sheet bars could be handledsatisfactorily.

The sintering as well as the substantial after-reduction were carriedout in furnace 3. The sintering comprised heat treatment in protectivegas at relatively high temperature, in order to impart to the sheet barssuch a strength, that they would withstand subsequent rolling withoutcrack formation. A further object was to remove the substantial part ofiron oxides contained in the pressed iron powder sheet bars by reactionwith the carbon content also contained in the sheet bars. q

The iron powder employed contained 1.1% FeO, 0.47 F and 0.36% C. Thecarbon-oxygen reaction commenced at about 600 C., but with a relativelylow velocity. At '1000'll00 C., however, the reaction velocity wasrelatively high. The reaction velocity depends partially on the densityof the sheet bars, in such a manner, that higher density renders a morerapid and complete reaction. The reaction was substantially notdependent on which protective gas atmosphere was used. During a furnacetreatment of 15 minutes at 1050 C., the carbon content in entirelyinactive argon could be reduced to 0.16% and the oxygen content (countedas FeO) to 0.4%. Experiments have proved, that mixtures containing COand methane had an advantageous effect on the afterreduction.

When applying a protective gas of the composition as follows.

Chemical composition: Percent CO E the C-content, for example, in thesintered sheet bars after a seven minute treatment at 1050 C. was 0.04%and the FeO-content 0.19%. By adjusting the CO and/or methane content inthe protective gas, it was possible to control the C-content in thefinal products.

The .density of the sintered sheet bars was slightly above 4, and theporosity about 45%. For obtaining a dense and crack-free product, theporous sheet bars were cold rolled. This type of rolling is calledcompression rolling. The tendency of crack formation was dependent onthe tensile strength and extension properties of the sheet bars, whichproperties in their turn depend on the density, compression pressure andsintering temperature. The risk of crack formation depends further onthe relation between the thickness of the sheet bars and the rolldiameter, in such a manner, that when this relation is small, i.e., withthin sheet bars and/or great roll diameter, the risk of cracks is small,even when the reduction chosen is considerable. Referring to the datawhich are actual in this example, namely: sheet bar thickness= mm.,density=4, sintering temperature=l050 C. and sintering time=10 minutes,a roll diameter of 700-900 mm. is suitable in rolling mill 4 at areduction of about 50%. When during some tests the sheet bars werecompressed to 2.6 mm. in one pass, i.e., a reduction of 48%, no crackformation was observed. The density increased from 4 to 7.1 g./cm. Thelateral spread was extremely low. The rolling speed was low, about 0.5m./sec.

After the compression rolling in mill 4, an intermediate annealing wascarried out in furnace 5, in order to increase "the strength of thematerial by re-crystallization and sintering for the subsequent coldrolling in ,mill 6. 5

The preferable temperature for the intermediate annealing depends on theconditions applied in the foregoing compression rolling. A temperatureof '840" C. was found suflicient.

In mill 6, three cold passes (17, 16 and 16%) were made.

In many cases the thin sheet is desired to be delivered in strips. Forrendering this possible according to the described embodiment of theinvention, the rolled billets must be welded together. This wasperformed in welding set 7. In order to prevent porosity variation inthe weld, the welding was carired out after the material was almostdense. The welding was followed by a heat treatment in furnace 8.

The rolling of the 1.5 mm. thick welded sheets to a cold rolled productwith a thickness of 0.8' mm. is preferably carried out by a series ofpasses with the reductions 21, 19 and 21%. This was'made in mill 9.After the cold rolling, dead annealing was performed in furnace 10 atabout 700 C. for about four hours at a relatively slow cooling. The saidannealing was followed by temper rolling with a reduction of 13% in mill11 with large roll diameters.

What we claim is:

1. Process for the production of rolled steel products which comprisesforming a ground sponge iron. powder having a substantial content ofiron oxide the bulk of which is exposed at the surfaces of the powderparticles, adjusting the carbon content of said iron powder to thestoichiometric equivalent of the iron oxide content thereof by theaddition of a member selected from the group consisting of pig iron andcementite, cold pressing said powder to sheet bars having a density ofat least 4 g./cm. sintering said bars at a temperature of 600l200 C.whereby the oxygen and carbon contents of the bars are substantiallyreduced, compression rolling the sintered bars to a density of at leastabout 7 gm./cm. annealing the compressed rolled bars and cold rollingthe annealed bars.

2. Process as defined in claim 1 in which the sintering is carried out.at a temperature within the range from 900 to 1150 C. in anon-oxidizing gas atmosphere.

3. Process as defined in claim 2 in which the gas atmosphere isreducing.

4. Process as defined in claim 1 in which the compression is effected bycold rolling.

5. Process as defined in claim 1 in which at least a part of the carboncontent of the iron powder is provided by the addition of cernentite.

6. Process as defined in claim 1 in which the sintering is carried outunder a protective carburizing gas.

7. Process as defined in claim 6 in which the protective carburizing gasis a member selected from the group consisting of carbon monoxide andmethane.

References Cited in the file of this patent UNITED STATES PATENTS2,222,251 Calkins et a1 Nov. 19, 194-0 2,686,118 Cavanagh Aug. 10, 19542,834,674 Silvasy et a1 May 13, 1958 2,839,397 Cavanagh June 17, 19582,922,223 Boughton et a1 Jan. 26, 1960 2,945,759 Shaw et a1. July 19,1960 FOREIGN PATENTS 570,345 Canada Feb. 10, 1959

1. PROCESS FOR THE PRODUCTION OF ROLLED STEEL PRODUCTS WHICH COMPRISESFORMING A GROUND SPONGE IRON POWDER HAVING A SUBSTANTIAL CONTENT OF IRONOXIDE THE BULK OF WHICH IS EXPOSED AT THE SURFACES OF THE POWDERPARTICLES, ADJUSTING THE CARBON CONTENT OF SAID IRON POWDER TO THESTOICHIOMETRIC EQUIVALENT OF THE IRON OXIDE CONTENT THEREOF BY THEADDITION OF A MEMBER SELECTED FROM THE GROUP CONSISTING OF PIG IRON ANDCEMENTITE, COLD PRESSING SAID POWDER TO SHEET BARS HAVING A DENSITY OFAT LEAST 4 G./CM.3, SINTERING SAID BARS AT A TEMPERATURE OF 600-1200*C.WHEREBY THE OXYGEN AND CARBON CONTENTS OF THE BARS ARE SUBSTANTIALLYREDUCED, COMPRESSION ROLLING THE SINTERED BARS TO A DENSITY OF AT LEASTABOUT 7 GM./CM.3, ANNEALING THE COMPRESSED ROLLED BARS AND COLD ROLLINGTHE ANNEALED BARS.